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13.1 History of Metric Monitoring While Chl-A Concentrations of The Nearshore Evaluation October 15, 2013 Version 10.e 13.1 History of Metric Monitoring While Chl-a concentrations of the plankton in the open-water or pelagic zone of Lake Tahoe has been sampled with some regularity (especially since 1984; TERC 2011), chlorophyll data from the nearshore or littoral plankton is much less common and only exists as part of limited, isolated studies. Early Chl-a data from the nearshore is available in McGaughey et al. (1963) for pelagic and nearshore sites around the lake. Between 1969 and 1975 the California-Nevada-Federal Joint Water Quality Investigations program collected Chl-a at a combined total of 15 nearshore stations (directly along the shoreline) and two deep-water limnetic sites (DWR 1971, 1972, 1973, 1974, 1975). Nearshore AGP was also measured in this program. Holm-Hansen (1976) made measurements of Chl-a in the water column at a central pelagic station in Lake Tahoe in the mid- 1970s as part of a research study of lake characteristics. Leigh-Abbott et al. (1978) studied chlorophyll a and temperature patterns along transects in the nearshore and offshore regions of the lake in 1977. Paerl et al. (1976) looked at adenosine triphosphate (ATP) and Chl-a levels in phytoplankton in Lake Tahoe from different depths, while Richerson et al. (1978) and Coon et al. (1980) investigated the processes involved with formation of the deep chlorophyll maximum in Lake Tahoe. Recent monitoring using in situ fluorescence to estimate Chl-a also has been done by the Desert Research Institute (DRI), both along the south shore and complete lake perimeters. The later were achieved through numerous cruises that circumnavigated the lake within the littoral zone, and which employed continuous measurements. Remote sensing data was used by Steissberg et al. (2010) in a detailed analysis of spatial and seasonal patterns of distribution of chlorophyll a in the upper euphotic zone of the nearshore. Recently (August 2011), researchers with the U.S. EPA, TERC and DRI, circumnavigated the lake as part of the PARASOL study (PARticulates And SOLutes in lakes) and took measurements of Chl-a (Kelly pers. comm.). In summary, much more effort has been put into measuring Chl-a in the open-water, pelagic portion of the Lake. Indeed, until the recent DRI continuous lake nearshore surveys, direct measurement of chlorophyll a in the nearshore or littoral has been very limited with the most comprehensive, historical monitoring coming from the early 1970s (DWR 1971-1975). 13.2 Monitoring Data Summary 13.2.1 Littoral Historic An example of historical information from the California-Nevada-Federal Joint Water Quality Investigations is chlorophyll a data at 12 nearshore sites reported for August 1971, May 1972, and August 1972 (DWR 1973), shown in Figure 13-1. Two pelagic stations (mid-lake north and mid-lake south) also were simultaneously sampled. The full study occurred from 1969–1974. Page 88 Nearshore Evaluation October 15, 2013 Version 10.e Individual measures of Chl-a in the littoral zone on these dates ranged from 0.09 mg/m3 (Camp Richardson) to 0.29 mg/m3 at Sunnyside (Fig. 13-1). The mean of the 12 nearshore stations was uniform over the three sampling dates; 0.17 mg/m3 (8/18/71), 0.16 mg/m3 (5/3/72) and 0.17 mg/m3 (8/3/72). The mean concentration for all littoral sites on all sampling dates was 0.17±0.05 (SD). The percentile values for these data were: 10th – 0.11 µg/L, 25th – 0.15 µg/L, 50th – 0.17 µg/L, 75th – 0.20 µg/L and 95th – 0.26 µg/L, with a maximum single value of 0.29 µg/L at Sunnyside in May 1972. Mean values near Taylor Creek, Rubicon Bay and Meeks Bay ranged from 0.13-0.16 µg/L, similar to the two limnetic sites – 0.12-0.13 µg/L. Sunnyside was the highest with a mean of 0.22 µg/L. The remaining sites ranged from 0.17-0.19 µg/L. For the entire five-year span of this monitoring program the mean±standard devation for all 140 Chl-a samples was 0.16±0.05 µg/L, with a spatially based range of means from 0.12-0.21 µg/L. There was a seasonal component to the distribution between 1969-1974, with a summer concentration (mean±standard deviation) of 0.12±0.06 µg/L (n=52) and a spring concentration of 0.21±0.06 µg/L. Sampling over the course of the study was largely confined to single collections in May and August. Figure 13-1. Concentrations of littoral and pelagic Chl-a in Lake Tahoe. Data are based on California- Nevada-Federal Joint Water Quality Investigations 1971-1972 (DWR 1973). CGP-Coast Guard Pier, KGB-Kings Beach, INC-Incline/Crystal Bay, ZPH-Zephyr Cove, TKS-Tahoe Keys, TLR-off Taylor Creek near Camp Richardson, RUB-Rubicon Bay, MKS-Meeks Bay, CHM-Chambers Landing and SUN-Sunnyside. Sites proceed clock-wise around the lake perimeter and samples were collected in direct proximity to the shoreline. Limnetic S-C refers to the lake center in the south with N-C was in the north. Refer to DWR (1973) for a detailed map of sampling sites. The ratio of littoral to pelagic Chl-a for all data was 1.4±0.4 (SD) µg/L (Figure 13-2). The individual sampling ratios were 1.0 for August 1971, 1.4 for May 1972 and 1.7 for August Page 89 Nearshore Evaluation October 15, 2013 Version 10.e 1972. However, 25 percent of all samples had a littoral:pelagic ratio less than 1.0, highlighting the fact that the littoral chlorophyll a was less than that found in the open water in some areas. The littoral:pelagic chlorophyll a ratio was lowest at near Taylor Creek and Meeks Bay (1.1 and 1.0, respectively). The ratio at Sunnyside was 1.8 while the remaining sites ranged from 1.3-1.5. Algal Growth Potential (AGP) tests were also conducted as part of the California- Nevada-Federal Joint Water Quality Investigations (1969–1974) using Chl-a as the measure of biomass increase during experimental incubations. These tests largely reflect the ability of phytoplankton to grow in ambient water as a function of the original biomass and nutrients. In very general terms, AGP can be used to help understand what the potential maximum biomass values could be. The ratios in Figure 13-3 represent Chl-a concentration after incubation for the littoral water divided by incubation for the pelagic water (littoral:pelagic), with data taken from the DWR study as done above for chlorophyll. A value of 1.0 denotes that the final Chl-a concentration in littoral samples after the experimental incubation was the same as Chl-a concentration in pelagic samples. Figure 13-2. Ratio of littoral versus limnetic (open-water) Chl-a concentrations. Data are based on California-Nevada-Federal Joint Water Quality Investigations 1971-1972 (DWR 1973). The horizontal line at 1.0 denotes that littoral Chl-a was identical to the pelagic concentrations. Refer to Figure 13-1 caption for site names. Page 90 Nearshore Evaluation October 15, 2013 Version 10.e Combining measurements from the example data set for August 1971, May 1972 and August 1972 (DWR 1973) the average ratio of littoral:pelagic AGP was 1.7±1.7 (SD); however, values varied between seasons and between years. A value of 2.0 for this ratio is the California state standard for Lake Tahoe (see Appendix B). Table 13-1 gives the final AGP derived Chl-a concentrations at the end of the incubation period for the littoral sites. These are shown as an indicator of what the possible maximum biomass may have been in the very early 1970s. Summarizing the full 1969–1974 DWR data set reveals that the nearshore:pelagic AGP ratio was <1.0 for 30.9 percent, 33.8 percent, and 37.5 percent, respectively, of experiments during spring (May), summer (August), and all seasons combined. A ratio between 1.0 and <2.0 was seen in 41.8 percent, 58.8 percent, and 51.7 percent of the tests during these same time periods. Also, for the same time periods the ratio was ≥2.0 for 27.3 percent, 7.4 percent and 10.7 percent of the tests (see Tables 13-5 to 13-7). 13.2.2 Littoral Recent DRI began to monitor nearshore Chl-a in the early 2000s with intermittent activity continuing in subsequent years. These provide useful information on the spatial distribution of nearshore Chl-a, but data are expressed in terms of relative units from sensor voltage values during in-situ continuous profiles around the lake. More recently, several samples were taken Figure 13-3. Ratio of littoral versus limnetic (open-water) Chl-a concentrations at the conclusion of AGP incubation periods. Data are based on California-Nevada-Federal Joint Water Quality Investigations 1971-1972 (DWR 1973). The horizontal line at 1.0 denotes that the final AGP littoral Chl-a were identical to the final AGP pelagic concentrations. A value of 2.0 is the California state standard for Lake Tahoe. Refer to Figure 13-1 caption for site names. Page 91 Nearshore Evaluation October 15, 2013 Version 10.e Table 13-1. Summary of final Chl-a values for the littoral sites at the conclusion of the AGP incubations. Data are based on California-Nevada-Federal Joint Water Quality Investigations 1971-1972 (DWR 1973). 1971-72 1971 1972 Mean (µg/L) 0.43 0.17 0.66 Stdev 0.45 0.07 0.51 Percentiles (µg/L) 10th 0.12 0.12 0.31 25th 0.15 0.13 0.35 50th 0.27 0.16 0.48 75th 0.48 0.21 0.68 95th 1.38 0.28 1.51 during a lake nearshore clarity circuit to calibrate relative voltage values to absolute (laboratory) measurement of chlorophyll concentration.
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